The present invention relates to a method for manufacturing a racquet frame for a sports racquet, which may be, for example, a tennis, squash, badminton, racquetball, soft tennis or paddle racquet. More particularly, the present invention relates to an improved method or manufacturing a racquet frame, which allows obtaining a composite frame structure forming the racquet frame. In a further aspect, the present invention relates to an improved racquet frame for a sports racquet having a composite frame structure and string holes formed therein.
Sports racquets have a head portion containing an interwoven string bed, a handle, and a shaft portion connecting the head portion to the handle.
In traditional racquets in composite materials, the frame is generally formed by placing in a mould a prepreg tube containing an inflatable bladder. The bladder is inflated, so as to make the prepreg tube to adhere to the mould walls and, at the same time, the mould is heated, so as to cure the prepreg tube. Holes for anchoring the ends of the strings are formed in the frame by drilling small string holes in the frame after the racquet is moulded. Each of those string holes (hereinafter referred to as “traditional string holes”) commonly accommodates a single string. Plastic grommet pegs, which are formed on grommet and bumper strips that extend along the outside surface of the frame, extend through the string holes to protect the strings from the sharp edges of the drilled holes.
Racquet frames, which are alternative with respect to those with traditional string holes, have been recently developed.
Co-owned PCT application WO 2004/075996 discloses a sports racquet, in which certain adjacent pairs of small string holes along the sides, tip, and throat bridge of the racquet are replaced by enlarged string port holes (hereinafter referred to as “string port holes”) having two inwardly facing string bearing surfaces, which are spaced apart by a distance corresponding to the distance between two contiguous main strings or cross strings. Preferably, the frame is formed of a double tube of carbon fibre-reinforced composite material (a so-called graphite frame), in which the string port holes are moulded into, as the racquet is pressure moulded. As a result of using two tubes, each forming one-half of the string port hole, the string port holes can have rounded edges and do not necessarily require the use of grommet pegs or strips. Also, in the regions between string port holes, the adjoining walls of the two tubes are fused together to form a stiffening wall inside the frame. The result is a racquet, which has improved torsional stiffness and lighter weight. The racquet is made in a mould having a mould cavity in the desired shape of the frame. The mould has two halves. A prepreg tube containing an inflatable bladder is placed in each mould half. Mould insert members, which have an outside surface in the desired shape of the string port holes, as well as pins to form traditional string holes are positioned between the two prepreg tubes and the mould is closed. The bladders are then inflated while the mould is heated to cure the composite resin. After removing the racquet frame from the mould, the mould insert members and the pins are removed leaving string port holes and traditional string holes, respectively.
Co-owned European patent application EP 06112486.3 discloses a sports racquet, in which a racquet frame with string port holes is formed using a single frame tube. In this case, a single mouldable structure is provided from a prepreg tube. Said structure contains a couple of co-axial prepreg inflatable bladders and a plurality of cross-channels, which transversally pass through the single tube structure at an intermediate region. The cross-channels have position and orientation corresponding to the position and orientation of the string port holes to be formed in the racquet frame. The tube structure is placed in a mould and mould members are inserted into the cross channels. The bladders are then pressurized and the tube structure conforms to the shape of the mould. The mould is heated, so that the tube structure cures. This manufacturing method is clearly intended to constitute an improvement, since a single moulding operation is adopted. This allows improving the quality of the racquet frames and obtaining a reduction of the overall manufacturing costs.
Although the above described manufacturing methods have shown to be effective for industrial manufacturing racquet frames with traditional holes or with string port holes, it has been seen that production costs are still relatively high, due to a multiplicity of factors.
First of all, these methods still entail a certain number of process steps, which is quite difficult to aggregate/reduce in order to save manufacturing time and costs. In fact, they are adopted inflating moulding techniques, which are relatively expensive and time consuming. For example, inflatable bladders have always to be positioned into the mouldable structure, so as to ensure upon pressurization the adhesion of the mouldable structure to the walls of the mould. This operation can be difficultly automated and very often it requires human intervention.
Further, the described manufacturing methods generally use mouldable tube structures and bladders, which are made of prepreg tubes with a high content of carbon fibres. It is known that carbon fibres are a relatively expensive material, the cost of which has been remarkably increasing in the recent years. This fact necessarily entails higher purchasing costs for providing the basic crude materials for manufacturing the racquet frame.
Moreover, the use of inflatable bladders makes it difficult to obtain additional holes or recesses (hereinafter referred to as “additional cavities”) on the racquet frame, which might be used for better accommodating bumpers or other plastic inserts, thereby improving the racquet frame structural performances. As for the traditional string holes these cavities might be drilled after the frame is formed. Unfortunately, the common practice has shown that drilling the string holes or the additional cavities is a kind of post-curing operation on the moulded racquet frame, which should be avoided since it may weaken the moulded frame, given the fact that the frame fibres are broken. In addition, this kind of operation requires time and that remarkably enhances the number of scrap frames.